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C/C++ overlay: Add basic Overlay.qll file #20909

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Merged
IdrissRio merged 5 commits into from
Nov 28, 2025
Merged

C/C++ overlay: Add basic Overlay.qll file #20909

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C/C++ overlay: Add basic Overlay.qll file
IdrissRio Nov 25, 2025
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C/C++ Overlay: Preserve entities that have at least one location in a…
IdrissRio Nov 26, 2025
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C/C++ overlay: Address review comments
IdrissRio Nov 26, 2025
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C/C++ overlay: Address review comments
IdrissRio Nov 28, 2025
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C/C++ overlay: Address review comments
IdrissRio Nov 28, 2025
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1 change: 1 addition & 0 deletions cpp/ql/lib/cpp.qll
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Original file line number Diff line number Diff line change
Expand Up @@ -74,3 +74,4 @@ import semmle.code.cpp.Preprocessor
import semmle.code.cpp.Iteration
import semmle.code.cpp.NameQualifiers
import DefaultOptions
private import semmle.code.cpp.internal.Overlay
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73 changes: 73 additions & 0 deletions cpp/ql/lib/semmle/code/cpp/internal/Overlay.qll
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@@ -0,0 +1,73 @@
/**
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* Defines entity discard predicates for C++ overlay analysis.
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*/

/**
* Holds always for the overlay variant and never for the base variant.
* This local predicate is used to define local predicates that behave
* differently for the base and overlay variant.
*/
overlay[local]
predicate isOverlay() { databaseMetadata("isOverlay", "true") }

overlay[local]
private string getLocationFilePath(@location_default loc) {
exists(@file file | locations_default(loc, file, _, _, _, _) | files(file, result))
}

/**
* An element with a single location. Discard if in a changed file.
*/
overlay[local]
abstract private class Discardable extends @element {
abstract string getFilePath();

predicate existsInBase() { not isOverlay() }

string toString() { none() }
}

overlay[discard_entity]
private predicate discardable(@element e) {
e = any(Discardable d | d.existsInBase() and overlayChangedFiles(d.getFilePath()))
}
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@igfoo igfoo Nov 26, 2025

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I think we're going to need to do something more complex for C. For example, if you have foo.h:

#define A 1

and bar.h:

#if A
int a;
#elif B
int b;
#endif

then if you change foo.h to

#define B 1

then bar.h's declarations change, but bar.h has not changed.

It gets even more complex when you might have different header variants, e.g. we might have one include or bar.h where A is defined, but a different include elsewhere where B is defined.

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@IdrissRio IdrissRio Nov 26, 2025

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Is this something we should do here, or should it happen when we detect which files changed?


/**
* An element with potentially multiple locations, e.g., variables, functions and types.
* Discard only if all locations are in changed files.
*/
overlay[local]
abstract private class MultiDiscardable extends @element {
abstract string getFilePath();

predicate existsInBase() { not isOverlay() }

string toString() { none() }
}

overlay[discard_entity]
private predicate multiDiscardable(@element e) {
e =
any(MultiDiscardable d |
d.existsInBase() and
forall(string path | path = d.getFilePath() | overlayChangedFiles(path))
)
}

overlay[local]
private class DiscardableVarDecl extends Discardable instanceof @var_decl {
override string getFilePath() {
exists(@location_default loc | var_decls(this, _, _, _, loc) |
result = getLocationFilePath(loc)
)
}
}

overlay[local]
private class DiscardableVariable extends MultiDiscardable instanceof @variable {
override string getFilePath() {
exists(@var_decl vd, @location_default loc | var_decls(vd, this, _, _, loc) |
result = getLocationFilePath(loc)
)
}
}